Laser optical apparatus

ABSTRACT

There is provided a structure for reducing optical loss in an optical apparatus (homogenizer) for making the intensity distribution of a laser beam uniform.  
     In a multi-cylindrical lens (a glass substrate having a multiplicity of cylindrical lenses formed thereon) used in a homogenizer, convex cylindrical lenses and concave cylindrical lenses are arranged alternately, and the boundaries between the cylindrical lenses have a smooth structure. This makes it possible to reduce scattering of beams that has occurred at the boundaries between the cylindrical lenses.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The invention disclosed in this specification relates to opticalapparatuses utilizing a laser such as apparatuses for performing anannealing process by means of irradiation with laser beams (laserannealing apparatuses) and, more particularly, to an laser annealingapparatus projecting beams with a large area which is capable ofproviding an uniform effect of irradiation. Such a laser annealingapparatus is used in semiconductor manufacturing steps and the like.

[0003] Laser beams with a large area are used in apparatuses includingexposure apparatuses for forming fine circuit patterns such assemiconductor circuits. Especially, ultraviolet laser beams are used forforming circuits with design rules on a sub-micron basis.

[0004] 2. Description of the Related Art

[0005] A techniques for crystallizing amorphous silicon films byirradiating them with laser beams has been known. Another known laserirradiation technique is to irradiate silicon films with laser beams inorder to recover them from damage to crystallinity thereof due to theimplantation of impurity ions and in order to activate the implantedimpurity ions. Such techniques are referred to as “laser annealingtechniques”.

[0006] A typical example of the latter technique is the annealing of thesource and drain regions of a thin film transistor. Those regions areannealed by irradiating them with laser beams after ions of impurities,typically phosphorus or boron, are implanted into those regions.

[0007] Such a process of irradiation with laser beams is characterizedby the fact that there is substantially no thermal damage to asubstrate.

[0008] The feature of giving no thermal damage to a substrate reduceslimitations on the materials to be subjected to such a process and isadvantageous, for example, in forming a semiconductor device on asubstrate made of glass or the like which has low heat resistance. Thisfeature is especially important in the fabrication of active matrixliquid crystal displays which recently have an increasing range ofapplication.

[0009] For an active matrix liquid crystal display, it is desirable touse a glass substrate from the viewpoint of cost and the requirement fora larger surface area.

[0010] A glass substrate can not withstand a heating process attemperatures as high as 600° C. or more or 700° C. or more. An effectivetechnique for avoiding this problem is to perform annealing after thecrystallization of a silicon film and the implantation of impurity ionsas described above by irradiating it with laser beams.

[0011] Even when a glass substrate is used, a method employingirradiation with laser beams results in substantially no damage to theglass substrate. It is therefore possible to use a glass substrate infabricating a thin film transistor having a crystalline silicon film.

[0012] There has been another proposal to use laser beams as a lightsource for forming fine circuit patterns taking advantage of the factthat laser beams are coherent light. Especially, the use of anultrasonic laser makes it possible to obtain fine patterns having sizesin sub-microns or smaller.

[0013] However, since laser beams have small beam areas when they aregenerated by a laser apparatus (hereinafter they are referred to as“source beams”), it is common to process a large surface area byscanning laser beams across it. This results in problems including lowuniformity of the effect of the process in a surface and a long periodof time required for the process. Especially, common source beams resultin a significant problem from the viewpoint of uniformity of the effectof processing when used as they are because they have non-uniformdistribution of light intensity.

[0014] Under such circumstances, a technique has been proposed whereinsource beams are processed into beams having highest possible uniformityand the beam size is changed in accordance with the shape of the surfacearea to be processed and the like. Common beam shape is rectangular orlinear shape. Such an arrangement makes it possible to perform uniformlaser annealing over a large surface area.

[0015]FIG. 1A shows an example of a laser irradiation apparatus in whichsource beams are processed. For example, an excimer laser is used as thelaser oscillator. Laser beams are oscillated by decomposingpredetermined gases by means of RF discharge to produce an excited statereferred to as “excimer state”.

[0016] For example, in a KrF excimer laser, an excited state KrF* isobtained by high voltage discharge using Kr and F as raw material gases.While this excited state is unstable as indicated by its duration in therange from several nanoseconds to several micro-seconds, KrF in theground state is more unstable. This results in inverted populationwherein the density in the excited state is higher than the density inthe ground state. As a result, induced radiation occurs, which makes itpossible to obtain laser beams having relatively high efficiency.

[0017] The laser oscillator is not limited to an excimer laser, andother pulse lasers or continuous lasers may be used. In general, pulselasers are appropriate for the purpose of achieving a high energydensity.

[0018] As shown in FIG. 1A, a source beam emitted by the laseroscillator is processed into an appropriate size by a beam expanderformed by a concave lens or a convex lens.

[0019] The beam then enters an optical device referred to as“homogenizer” which includes at least one lens device (multicylindricallens) having a multiplicity of cylindrical lenses (generally in aparabolic configuration). As shown in FIG. 1B, a conventionalmulti-cylindrical lens includes a plurality of cylindrical lenses 1through 5 (which are all convex lenses) formed on a single sheet ofglass.

[0020] In general, two multi-cylindrical lenses are used and arranged sothat they are perpendicular to each other. Obviously, the number of themulti-cylindrical lens may be one or three or more. When onemulti-cylindrical lens is used, the non-uniformity of a source beam inone direction is dispersed. When two or more multi-cylindrical lensesare formed in the same direction, the same effect as increasing thenumber of the cylindrical lenses can be achieved.

[0021] When a beam passes through the multi-cylindrical lens, the beamcan be converted into a uniform beam having a distributed energydensity. The principle behind this will be described later. Thereafter,the beam is processed by a converging lens into a desired shape or, ifneeded, deflected by a mirror to be projected upon a sample (see FIG.1A).

[0022] A description will now be made on the principle of a conventionalhomogenizer (multi-cylindrical lens) and a problem of the same which isthe problem to be solved by the invention. In order to avoidcomplication, discussion on an optical basis will be focused on only onesurface. Laser beams that have passed through a multi-cylindrical lensare as shown in FIG. 2A.

[0023] Here, the multi-cylindrical lens L includes five convexcylindrical lenses 1, and the beam incident upon each of the cylindricallenses is refracted by the cylindrical lens. After being converged at afocal point, the beams are diffused. This process results in a region inwhich all of the beams that pass through the respective cylindricallenses are mixed (mixed region).

[0024] Let us assume here that the distribution of the optical intensityof the beams is polarized, resulting in differences in the intensity ofthe beams incident upon the respective cylindrical lenses. In the mixedregion, however, such polarization is scattered because the beams thatpass through the respective cylindrical lenses are mixed. That is, theoptical intensity is made uniform. It is thus possible to obtain beamshaving less distribution of optical intensity (see FIG. 2A).

[0025] When we look at the paths of the beams that pass through themulti-cylindrical lens, the beams can be regarded as beams emitted frompoint light sources F (i.e., focal points) arranged at equal intervals(distances “a”) (see FIG. 2B).

[0026] The same effect can be achieved by providing a convex cylindricallens 1 ₁ on one side of a glass substrate and a convex cylindrical lens1 ₂ on the other side at an interval “a”. In FIG. 3A, the path of a beamthat has passed through the cylindrical lens 1 ₁ is indicated by thesolid line, and the path of a beam that has passed through thecylindrical lens 1 ₂ is indicated by the broken line. In this case, amixed region is obtained as in the case shown in FIG. 2A (see FIG. 3A).

[0027] When we look at the paths of the beams that pass through themulti-cylindrical lens, as shown in FIG. 3B, the beams can be regardedas beams emitted from two kinds of point light sources F₁ and F₂ (i.e.,focal points) (see FIG. 3B).

[0028] In the case of the conventional multi-cylindrical lens asdescribed above, since there is an angle at the ends of each cylindricallens (boundaries between itself and other cylindrical lenses), beamshave been scattered at such regions, which has resulted in optical loss.This means that the laser beams can not be effectively utilized and thebeam intensity is reduced.

SUMMARY OF THE INVENTION

[0029] The present invention has been conceived taking theabove-described problem into consideration. A multicylindrical lensaccording to the present invention is characterized in that not onlyconvex cylindrical lenses but also concave cylindrical lenses are used;the convex cylindrical lenses and concave cylindrical lenses arealternately arranged; and the cylindrical lenses smoothly continue toeach other.

[0030] According to a first aspect of the invention, a multicylindricallens having the above-described configuration is provided in ahomogenizer which is inserted and used between a laser oscillator and anobject to be irradiated in an apparatus for forming a laser beam havinga linear or rectangular beam shape.

[0031] According to a second aspect of the invention, in a laser opticalapparatus having a laser oscillator and a homogenizer to which a laserbeam emitted by the laser oscillator is input, a multi-cylindrical lensused in the homogenizer have the above-described configuration.

[0032] In the above-described multi-cylindrical lens, the state of thecurved surface of the convex cylindrical lenses may be the same as thestate of the curved surface of the concave cylindrical lenses. Further,at least two multi-cylindrical lenses may be provided in thehomogenizer, and the two multi-cylindrical lenses may be provided indirections perpendicular to each other.

[0033] The configuration of a multi-cylindrical lens according to theinvention is as shown in FIG. 1C. Specifically, while any conventionalmulti-cylindrical lens has been formed by convex cylindrical lenses 1through 5 (see FIG. 1B), concave cylindrical lenses 2 and 4 are providedbetween the convex cylindrical lenses 1 and 3 and between the convexcylindrical lenses 3 and 5, respectively, and the boundaries betweenthose cylindrical lenses are smooth and continuous (see FIG. 1C).

[0034] The cylindrical lenses can be smoothly connected to each other bymaking the curvature (the shape of the curved surface) of a concavecylindrical lens identical to a convex cylindrical lens adjacentthereto. When the cylindrical lenses are parabolic lenses, there isprovided a structure which is a combination of parabolic surfaces havingdifferent orientations. The paths of laser beams in such amulti-cylindrical lens are as shown in FIGS. 4A and 4B when illustratedin the same manner as in FIGS. 2A and 2B and FIGS. 3A and 3B.

[0035] Here, the multi-cylindrical lens L includes three convexcylindrical lenses 1 ₁ and two concave cylindrical lenses 1 ₂ as shownin FIG. 1C. Beams incident upon the convex cylindrical lenses arediffused after being converged at focal points. On the other hand, beamsincident upon the concave cylindrical lenses are simply diffused as ifthey are diffused from certain points. As a result, there is obtained aregion where all of the beams that pass through the cylindrical lensesare mixed (mixed region).

[0036] The above-described effect can be similarly achieved even whenthe convex cylindrical lenses and the concave cylindrical lenses havedifferent curvatures (a factor that determines a focal distance or theshape of the curved surface of a lens). In addition, optical loss can bereduced in a multi-cylindrical lens having such a configuration becausethere is no structure that scatters beams (an unsmooth region such as aprotrusion) (see FIG. 4A).

[0037] When we look at the paths of the beams that pass through themulti-cylindrical lens, the beams can be regarded as beams emitted fromtwo kinds of point light sources F₁ and F₂ (i.e., focal points) as thoseshown in FIG. 3B (see FIG. 4B)

[0038] When the convex cylindrical lenses and the concave cylindricallenses have the same curvature, the boundaries of the paths of the beamsthat pass through the concave cylindrical lenses pass through the focalpoints F₁ of the adjacent convex cylindrical lenses. A descriptionfollows on this point. When convex cylindrical lenses and the concavecylindrical lenses have the same curvature, the angle of convergence bythe former (the angle at which the beams are diffused after passingthrough the focal points) is the same as the angle of diffusion by thelatter when collimated beams are incident.

[0039] Specifically, when the focal distance of a convex cylindricallens 1 ₁ is represented by x as shown in FIG. 5A, collimated beams thatpass through a concave cylindrical lens 1 ₂ can be regarded as beamsemitted from a point F₂ which is at the distance x toward the side atwhich the beams enter. Let us consider the path F₁A of a beam thatpasses through the lower end of the convex cylindrical lens 1 ₁ and thepath F₂A of a beam that passes through the upper end of the concavecylindrical lens 1 ₂. Then, since the angles of diffusion andconvergence of the beams that pass through those lenses are the same,the line F₂A overlaps the line F₁A. That is, the boundary of the path ofthe beam passing through the concave cylindrical lens passes through thefocal point F₁ of the convex cylindrical lens adjacent thereto (FIG.5B).

BRIEF DESCRIPTION OF THE DRAWINGS

[0040]FIG. 1A is a schematic view of an optical system of a laserirradiation apparatus.

[0041]FIG. 1B is a schematic view of a conventional multi-cylindricallens.

[0042]FIG. 1C is a schematic view of a multi-cylindrical lens accordingto the present invention.

[0043]FIGS. 2A and 2B are schematic views showing beam paths in aconventional multi-cylindrical lens.

[0044]FIGS. 3A and 3B are schematic views showing beam paths in aconventional multi-cylindrical lens.

[0045]FIGS. 4A and 4B are schematic views showing beam paths in amulti-cylindrical lens according to the present invention.

[0046]FIGS. 5A and 5B are views showing beam paths of convex lenses andconcave lenses.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT

[0047] An optical system of an embodiment of the present invention willnow be described. A laser irradiation apparatus according to the presentembodiment has the same basic configuration as that shown in FIG. 1A.The shape of a laser beam before incidence upon a homogenizer isexpressed by 6 cm×5 cm. In this embodiment, a multi-cylindrical lens isused as the homogenizer. Here, only the multi-cylindrical lens will bedescribed.

[0048] In the configuration shown in this embodiment, themulti-cylindrical lens is formed by arranging six concave cylindricallenses (having a width of 5 mm) and five convex cylindrical lenses(having a width of 5 mm) alternately to divide an incident beam intoabout ten beams. The length of the cylindrical lenses in thelongitudinal direction thereof is 7 cm. The multi-cylindrical lens ismade of quartz.

[0049] In the present embodiment, the length of a liner laser beam thatis finally projected is 12 cm in the longitudinal direction thereof andthe width is 0.5 mm. As a result, the laser beam is enlarged by a factorof 2 in the longitudinal direction and is reduced by a factor of 100 inthe direction perpendicular thereto after the laser beam passes throughthe homogenizer. All of the convex cylindrical lenses and concavecylindrical lenses are spherical lenses and have the same curvature. Thefocal distance of the convex cylindrical lenses is 5 cm(see FIG. 1A).

[0050] The use of the invention disclosed in this specification makes itpossible to obtain a uniform laser beam having a large area required ina laser process used for the fabrication of a semiconductor device andthe like.

[0051] It should be understood that the foregoing description is onlyillustrative of the invention. Various alternatives and modificationscan be devised by those skilled in the art without departing from theinvention. Accordingly, the present invention is intended to embrace allsuch alternatives, modifications and variances which fall within thescope of the appended claims.

What is claimed is:
 1. A laser optical apparatus for forming a laserbeam having a linear or rectangular beam shape comprising: a homogenizerinserted and used between a laser oscillator and an object to beirradiated, wherein at least one multi-cylindrical lens used in saidhomogenizer comprises a plurality of convex cylindrical lenses andconcave cylindrical lenses arranged alternately, and one of said convexcylindrical lenses and one of said concave cylindrical lenses adjacentto each other are continuous without having an angle.